The invention relates to check valves and more particularly to valves for preventing back flow.
Check valves are provided in a conduit system to prevent the backwards flow of liquid, e.g. in the event of a reduction or reversal of a supply pressure In a typical water piping system in a home or other building, check valves are used to prevent back flow of potentially contaminated water from the building water system (e.g. water from a wash tub containing detergent or other cleaning agents, or water from a lawn or garden hose contaminated with fertilizer or other chemicals) into the water main, as may occur, e.g., due to a sudden loss of pressure in the water main. Different valve systems have been designed for the purpose of assuring that a drop in supply pressure will result in the closing of the check valve in order to prevent back flow.
In one prior art back flow valve, the sealing plug is weighted to bias the valve toward the closed position. Forward flow of a liquid under normal pressure forces the valve into open position, but upon pressure reduction, the weight aids in closing the valve. A weight biased valve typically does not close quickly enough to avoid flow reversal which slams the valve shut and creating a noisy, hammering sound.
Referring to FIG. 1, another prior art check valve 1 has a sealing plug 2 mounted upon a scissor assembly 4. Springs 6 are attached at both sides of the scissor assembly 4 between the scissor hinge arms 8, 8' which are generally perpendicular to the plane P of the sealing plug 2 in closed position. The springs are disposed generally at a 45 degree angle to the flow of liquid, the bias of springs 6 serving to urge the scissor assembly 4 toward a compressed configuration, thereby forcing the sealing plug 2 against the inlet 9 to positively close the valve. The forward flow of liquid (arrow F) under normal pressure opens the valve.